1. Field of the Invention
The present invention relates to a carbon fiber-reinforced metallic material and a method of producing the same.
2. Description of the Prior Art
One method of producing such a metallic material which is conventionally known comprises forming a fiber shaped article having a predetermined bulk density from carbon fibers, preheating the fiber shaped article in a nonoxidative atmosphere in a preheating furnace to a predetermined temperature in order to enhance the filling property of the fiber shaped article with a molten metal and then, withdrawing the fiber shaped article out of the preheating furnace into the atmosphere to place it into a mold and pouring a molten metal into the mold to fill the fiber shaped article with the molten metal and solidify the molten metal.
However, the above method is accompanied by a problem that the carbon fibers after being preheated immediately starts to be oxidized in the atmosphere, resulting in a substantially reduced tensile strength, and for this reason, it is impossible to achieve fully the reinforcement of a metallic material with fibers.
A method is also known in which a magnesium alloy in the form of a melt is filled into a fiber shaped article formed from carbon fibers and then solidified by utilizing a high pressure solidification casting process. In such a case, an alloy for cast products is employed as the magnesium alloy.
With this method, however, problems are encountered. The alloy for cast products contains aluminum in a large amount of 5 to 10% by weight and therefore, such aluminum easily reacts with carbon fibers to form a large amount of an embrittled layer, thus considerably reducing the tensile strength of the carbon fiber.
In addition, with the increase in aluminum content, the formation is increased of the layer of an intermetallic compound such as Al.sub.12 Mg.sub.17 in the matrix. This layer has a tensile strength of 8 to 12 kg/mm.sup.2 and an elongation of 0.3 to 0.5% and thus, is very brittle. Moreover, the layer tends to segregate on the carbon fiber surface or in the vicinity thereof, resulting in that a notch is formed due to the initial fracture of such layer and with the progression of the formation, the tensile strength of the resultant material is remarkably reduced.